The present invention generally relates to a circuit for generating a potential, and more particularly relates to a common mode bias generator that can supply a required minimum amount of current without dissipating it unnecessarily.
As LSIs have been further miniaturized over the past few years, it has become more and more necessary for analog circuits for the LSIs to operate with even lower power dissipated or to further reduce their sizes. This is because the final value of an LSI product greatly depends on the performance of the analog circuits. Accordingly, the more stringent the requirements imposed on those analog circuits, the more pressing the need for reduction in power dissipation of the circuits. For example, particularly when high-speed transmission should be realized or a wide range of terminal potentials should be handled as in IEEE 1394-compliant systems, analog circuits such as drivers, operational amplifiers and comparators must operate to the limit of their abilities under the current circumstances.
More specifically, if an operational amplifier should supply a large current, then the size of output transistors on the last stage of the amplifier usually has to be increased. This is because if the size of the output transistors is not large enough, then the amplifier cannot supply current in the required amount and eventually the potential at the output node thereof adversely decreases. Thus, that is an inevitable choice to make even though the overall size of the amplifier needs to be reduced.
For example, suppose a TPBIAS circuit should provide an output potential at a certain level and supply a current of about-3 mA to about+25 mA. The circuit, complying with the IEEE 1394, generates a common mode bias voltage at a high-speed differential output node coupled to a so-called "twisted pair" cable. In that case, the sizes of transistors on the last stage of the circuit (e.g., PMOS transistors, in particular) should be increased to such an extent that the circuit can supply the maximum current of 25 mA.
However, as for a common mode bias generator such as that defined by the IEEE 1394, the amount of current to be supplied by the circuit changes incessantly. In other words, the circuit does not always have to supply the maximum amount of current. Accordingly, if the circuit has been designed to always supply the maximum amount of current, then the circuit will waste too much current in vain even while the circuit has to supply no currents (i.e., the amount of current to be supplied is 0 mA). This is because the last-stage transistors of the relatively large size also spend too much current even in such an idling state.